1. Field of the Invention
This invention relates to a device for recording and subsequently reproducing desired portions of an ECG signal produced by a heart prior to and during the occurrance of various disturbances in cardiac electrical activity.
2. Description of the Prior Art
During the past several decades, coronary heart disease has come to occupy the first position among the causes of death in the developed areas of the world. Although the precise cause of sudden death in coronary heart disease has not yet been entirely clarified, the available evidence permits the medical field to ascribe death in the majority of these cases to a grave disturbance in cardiac electrical activity resulting in ventricular fibrillation.
While it is not possible to predict with unerring exactness which patient suffering from coronary heart disease will be the victim of sudden death, several high risk groups of patients can be recognized. For example, patients who have experienced myocardial infarction, even though they may be surviving in good health, run a substantial risk of dying suddently, a risk several times greater than that associated with the general population. Further, if patients with myocardial infarction have a history of serious ventricular arrhythmias and/or of cardiac arrest, or if evidence of persistent myocardial irritability is present, it may be logically assumed that the risk of sudden death is increased substantially. Patients like those described above would greatly benefit from an automatic, standby or demand defibrillator.
Another recognizable class of patients particularly in need of an automatic defibrillator is the class composed of those who have not shown prior histories of myocardial infarction but who show severe symptoms of coronary heart disease, such as ventricular arrhythmias resistant to medical treatment or angina pectoris.
Finally, there are scores of individuals walking the streets today who experience recurring episodes of atrial fibrillation, atrial flutter, or tachycardia. While not life-threatening, these supra-ventricular arrhythmias can become debiliting and lead to complications, and hence require treatment when present. Such individuals require frequent electrical or pharmacological conversion under the care of their physicians to return their hearts to normal sinus rhythm.
Great strides are presently being made to develop an automatic, fully implantable ventricular defibrillator. See, for example, U.S. Pat. Nos. Re. 27,652 and 27,757, where the first concept of the automatic implantable ventricular defibrillator is described. Recent advances have also been made in enhancing the reliability of fibrillation detectors. In this latter regard, see copending U.S. Pat. Applications Ser. Nos. 878,005 and 878,006, each filed on Feb. 15, 1978. Furthermore, as outlined in copending U.S. Pat. Application Ser. No. 801,300, filed on May 27, 1977, steps have been taken to improve the reliability of the implanted defibrillator by the provision of circuitry which interrogates the implanted electronics to verify proper operation before a defibrillating shock is delivered.
Notwithstanding the substantial steps which have been taken to develop the automatic, fully implantable defibrillator and to ensure the operation of the sensing and defibrillating circuitry, it must not be forgotten that the implantable defibrillator is in its infancy. Accordingly, there is a current need for data which either verifies the accuracy or which uncovers the failings of the sensing and defibrillating circuits. Specifically, there is a need for a practical device capable of providing data by recording and subsequently reproducing desired portions of an electrocardiogram (ECG) signal produced by a heart prior to and during the occurrence of various disturbances in cardiac electrical activity. With such a device, not only could the operation of the implanted defibrillator be verified, but valuable information about the patient's heart activity prior to and during cardiac arrhythmias could be obtained. Furthermore, there is a need for a practical device which could be worn by a patient to monitor heart activity even in the absence of an implanted defibrillator.
It is toward the object of meeting the foregoing needs that the present invention is directed.